High energy proton irradiation is a recent development in the treatment of cancer and other diseases. A high-energy proton beam is used to deliver radiation as precisely as possible to a tumor volume, minimizing radiation delivery to healthy tissue.
One of the challenges in so using a proton beam is the precise positioning of the patient (and tumor). It has been suggested that the proton beam itself can be used to position the tumor, both before and during treatment. Romero, J. L., et al., 1995 Nucl. Instr. Meth., A 356, 558. If a high energy proton beam is used, such that the protons are not stopped inside the patient, the protons can be used to generate a density image of the patient. Romero, et al., suggested generating such an image through the use of wire chambers to determine proton location and layers of plastic scintillators monitored by photomultiplier tubes to determine proton energy. This method assumes, however, that a proton's energy is absorbed in part in the patient but that its direction remains unchanged. In fact, protons are scattered and often exit the patient in a direction different from the incident direction, thereby degrading the image quality. If it is possible to determine a proton's direction after exiting the patient, scattered protons can be identified and discarded. Other protons that are also included in radiation "background" can also be identified and discarded. The present invention allows the precise tracking, i.e. the determination of the location, energy and direction, of a proton.
The present invention is a method and apparatus for imaging objects. It does so by the 3-dimensional tracking of protons that have passed through the object to be imaged. The tracking is accomplished by gathering and analyzing images of the ionization tracks of the protons in a closely packed stack of scintillating fibers. The fibers are arranged in stacked layers with the fibers in a given layer orthogonal to those in the layers immediately adjacent.
A similar arrangement of scintillating fibers has been used to determine the location of the incidence of ionizing radiation. No suggestion was made, however, for its use in determining the direction or energy of the ionizing radiation. Fenyves U.S. Pat. No. 5,103,098.
A similar arrangement of scintillating fibers has also been proposed for measuring solar neutrons in the 20-200 MeV range. Measurements of the solar neutrons are proposed by imaging the ionization tracks of protons produced as a result of the elastic scattering of the solar neutrons off hydrogen within the scintillating fibers. Ryan, et al., 1997 SPIE, Vol. 3114, 514. No suggestion was made, however, for its use in any type of imaging of objects through which protons have passed, the protons being tracked having been produced within the scintillating fibers of the detector.
The present invention can be used to generate density images of patients for diagnosis and treatment. It can also be used to generate density images of other objects. It can also be used, for example, to measure the thickness at various points of objects of uniform composition.
The present invention has significant advantages as opposed to existing systems of imaging using protons. As noted above, it precisely tracks the protons, determining the direction, as well as the location and energy, of each proton that has passed through the object to be imaged. This capability allows the effective identification and substraction of radiation background, permitting a higher signal to noise ratio and higher contrast images. The present invention also permits real-time imaging.